Masked digital signatures

ABSTRACT

The present invention relates to digital signature operations using public key schemes in a secure communications system and in particular for use with processors having limited computing power such as ‘smart cards’. This invention describes a method for creating and authenticating a digital signature comprising the steps of selecting a first session parameter k and generating a first short term public key derived from the session parameter k, computing a first signature component r derived from a first mathematical function using the short term public key, selecting a second session parameter t and computing a second signature component s derived from a second mathematical function using the second session parameter t and without using an inverse operation, computing a third signature component using the first and second session parameters and sending the signature components (s, r, c) as a masked digital signature to a receiver computer system. In the receiver computer system computing a recovered second signature component s′ by combining a third signature component with the second signature component to derive signature components (s′, r) as an unmasked digital signature. Verifying these signature components as in a usual ElGamal or ECDSA type signature verification.

This is a continuation of U.S. application Ser. No. 08/966,702, filed Nov. 10, 1997.

This invention relates to a method of accelerating digital signature operations used in secure communication systems, and in particular for use with processors having limited computing power.

BACKGROUND OF THE INVENTION

One of the functions performed by a cryptosystem is the computation of digital signatures that are used to confirm that a particular party has originated a message and that the contents have not been altered during transmission. A widely used set of signature protocols utilizes the ElGamal public key signature scheme that signs a message with the sender's private key. The recipient may then recover the message with the sender's public key. The ElGamal scheme gets its security from calculating discrete logarithms in a finite field. Furthermore, the ElGamal-type signatures work in any group and in particular elliptic curve groups. For example given the elliptic curve group E(F_(q)) then for P ∈E(F_(q)) and Q=aP the discrete logarithm problem reduces to finding the integer a. Thus these cryptosystems can be computationally intensive.

Various protocols exist for implementing such a scheme. For example, a digital signature algorithm DSA is a variant of the ElGamal scheme. In these schemes a pair of correspondent entities A and B each create a public key and a corresponding private key. The entity A signs a message m of arbitrary length. The entity B can verify this signature by using A's public key. In each case however, both the sender, entity A, and the recipient, entity B, are required to perform a computationally intensive operations to generate and verify the signature respectively. Where either party has adequate computing power this does not present a particular problem but where one or both the parties have limited computing power, such as in a ‘smart card’ application, the computations may introduce delays in the signature and verification process.

Public key schemes may be implemented using one of a number of multiplicative groups in which the discrete log problem appears intractable but a particularly robust implementation is that utilizing the characteristics of points on an elliptic curve over a finite field. This implementation has the advantage that the requisite security can be obtained with relatively small orders of field compared with, for example, implementations in Z_(p)* and therefore reduces the bandwidth required for communicating the signatures.

In a typical implementation of such a digital signature algorithm such as the Elliptic Curve Digital Signature Algorithm (ECDSA) a signature component s has the form: s=k ⁻¹(e+dr) mod n

where:

-   -   d is a long term private key random integer of the signor;     -   Q is a public key of the signor derived by computing the point         Q=dP;     -   P is a point (x, y) on the curve which is a predefined parameter         of the system;     -   k is a random integer selected as a short term private or         session key, and has a corresponding short term public key R=kP;     -   e is a secure hash, such as the SHA-1 hash function of a         message; and     -   n is the order of the curve.

In this scheme the signor represents the x coordinate of the point kP as an integer z and then calculates a first signature component r=z mod n. Next, the second signature component s above is calculated. The signature components s and r and a message M is then transmitted to the recipient. In order for the recipient to verify the signature (r, s) on M, the recipient looks up the public key Q of the signor. A hash e′ of the message M is calculated using a hash function H such that e′=H(M). A value c=s⁻¹ mod n is also calculated. Next, integer values u₁ and u₂ are calculated such that u₁=e′c mod n and u₂=rc mod n. In order that the signature be verified, the value u₁P+u₂Q must be calculated. Since P is known and is a system wide parameter, the value u₁P may be computed quickly. The point R=u₁P+u₂Q is computed. The field element x of the point R=(x₁,y) is converted to an integer z, and a value v=z mod n is computed. If v=r, then the signature is valid.

Other protocols, such as the MQV protocols also require similar computations when implemented over elliptic curves which may result in slow signature and verification when the computing power is limited. The complexity of the calculations may be explained by observing a form of the elliptic curve. Generally, the underlying elliptic curve has the form y²+xy=x³+ax+b and the addition of two points having coordinates (x₁,y₁) and (x₂,y₂) results in a point (x₃,y₃) where:

$x_{3} = \left\{ {{\left( \frac{y_{1} \oplus y_{2}}{x_{1} \oplus x_{2}} \right)^{2} \oplus \frac{y_{1} \oplus y_{2}}{x_{1} \oplus x_{2}} \oplus x_{1} \oplus x_{2} \oplus {a\mspace{20mu}\left( {P \neq Q} \right)y_{3}}} = \left\{ {\left( \frac{y_{1} \oplus y_{2}}{x_{1} \oplus x_{2}} \right) \oplus \left( {x_{1} \oplus x_{3}} \right) \oplus x_{3} \oplus {y_{1}\mspace{25mu}\left( {P \neq Q} \right)}} \right.} \right.$ The doubling of a point i.e. P to 2P, is performed by adding the point to itself so that

${y_{3 =}\left\{ {x_{1}^{2} \oplus \left( {x_{1} \oplus \frac{y_{1}}{x_{1}}} \right)} \right\} x_{3}} \oplus x_{3}$ $x_{3} = {x_{1}^{2} \oplus \frac{b}{x_{1}^{2}}}$

It may be seen in the above example of the ECDSA algorithm that the calculation of the second signature component involves at least the computation of an inverse. Modulo a number the generation of each of the doubled points requires the computation of both the x and y coordinates and the latter requires a further inversion. These steps are computationally complex and therefore require either significant time or computing power to perform. Inversion is computationally intensive, and generally performed within a secure boundary where computational power is limited thus it would be advantageous to perform such calculations outside the secure boundary, particularly where computational power is more readily available. This however cannot be done directly on the ECDSA signature scheme without potentially compromising the private key information. Therefore there exists a need for a method of performing at least part of a signature operation outside a secure boundary while still maintaining an existing level of security in current signature schemes.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method and apparatus in which at least some of the above disadvantages are mitigated.

This invention seeks to provide a digital signature method, which may be implemented relatively efficiently on a processor with limited processing capability, such as a ‘smart card’ or the like.

In general terms, the present invention provides a method and apparatus in which signature verification may be accelerated.

In one aspect, the present invention provides a method for verifying a signature for a message m in a data communication system, established between a sender and a recipient, said sender generating masked signature components (r, s, c), where r is an integer derived from a coordinate of a first short term public key kP, s is a signature component derived by binding a second short term private key, the message m and short and long term private keys, and c is a second signature component obtained by combining said first and second short term private keys, the method comprising the steps of having a verifier: obtain a pair of signature components ( s, r), the component s being derived from first and second signature components generated by a signor; recovering a coordinate pair (x₁,y₁) corresponding to said first short term public key kP, using the pair ( s, r) and the message m; calculate a signature component r′ from one of the coordinate pairs; and verify the signature if r′=r.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a communication system; and

FIG. 2 is a flow chart showing a signature algorithm.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring therefore to FIG. 1, a data communication system 10 includes a pair of correspondents, designated as a sender 12, and a recipient 14, who are connected by a communication channel 16. Each of the correspondents 12,14 includes an encryption unit 18,20 respectively that may process digital information and prepare it for transmission through the channel 16 as will be described below. The sender is the party signing a message m to be verified by the recipient. The signature is generally performed in the encryption unit 18 and normally defines a secure boundary. The sender could be a ‘smart card’, a terminal or similar device. If for example the signor is a ‘smart card’, it generally has limited processing power. However, the ‘smart card’ is typically used in conjunction with a terminal 22 which has at least some computing power. The ‘smart card’ is inserted into a terminal 22 which then forwards digital information received from the ‘smart card’ 12 along the channel 16 to the recipient 14. The terminal may preprocess this information before sending it along the channel 16.

In accordance then with a general embodiment, the sender assembles a data string, which includes amongst others the public key Q of the sender, a message m, the sender's short-term public key R and a signature S of the sender. When assembled the data string is sent over the channel 16 to the intended recipient 18. The signature S is generally comprised of one or more components as will be described below with reference to a specific embodiment and according to a signature scheme being implemented by the data communication system.

The invention describes in a broad aspect a signature algorithm in which the private key is masked to generate masked signature components which may then be converted to a regular signature prior to the verification of the signature.

Referring to FIG. 2, it is assumed that E is an elliptic curve defined over Fq, P is point of prime order n in E(Fq), d is the senders private signature key, such that 2≦d≦n−2, Q=dP is the senders public verification key and m is the message to be signed. It is further assumed these parameters are stored in memory within a secure boundary as indicated by block 30. For example if the sender is a ‘smart card’, then that would define the secure boundary while for example the terminal in which the ‘smart card’ was inserted would be outside the secure boundary as indicated at 32. The first step is for the sender to sign the message m. The sender computes a hash value e=H(m) of the message m, where H is typically a SHA-1 hash function. A first statistically unique and unpredictable integer k, the first short term private key, is selected such that 2≦k≦(n−2). Next a point (x₁,y₁)=kP is computed. The field element x₁ of the point kP is converted to an integer x₁ and a first signature component r= x₁ (mod n) is calculated. A second statistically unique and unpredictable integer the second short-term private key is selected such that 2≦t≦(n−2). Second and third signature components s=t(e+dr)(mod n) and c=tk (mod n) respectively are also computed as indicated. This generates the masked ECDSA signature having components (r,s,c). This masked ECDSA signature (r,s,c) may be converted to regular ECDSA signature ( s, r) by computing s=c⁻¹s mod n. The ECDSA signature of the sender 12 is then s and r. The signature ( s, r) can then be verified as a normal ECDSA signature as described below. Thus the sender can either forward the masked ECDSA signature (s,r,c) to the verifier where the verifier can do the conversion operation to obtain the signature ( s, r) prior to the verification operation or the sender can perform the conversion outside the secure boundary, as for example in a terminal and then forward the DSA signature ( s, r) to the verifier.

Once the recipient has the signature components ( s, r), the recipient can verify the signature by calculating a hash value e=H(m) where this the hash function of the signor and known to the verifier of the message m and then computing u= s ⁻¹e mod n and v= s ¹r mod n. Thus the point (x₁,y₁)=uP+vQ may now be calculated. If (x₁,y₁) is the point at infinity then the signature is rejected. If not however the field element x₁ is converted to an integer x₁ . Finally the value r′= x₁ mod n is calculated. If r′=r the signature is verified. If r′≠r then the signature is rejected.

Thus it may be seen that an advantage of the masked ECDSA is that modular inverse operation of the normal ECDSA is avoided for the masked signing operation. As stated earlier this is very useful for some applications with limited computational power. The masked signature to ECDSA signature conversion operation can be performed outside the secure boundary protecting the private key of the sender. For example if the sender was a ‘smart card’ that communicated with a card reader then this operation could be performed in the ‘smart card’ reader. Alternatively the masked signature can be transmitted to the verifier, and the verifier can do the conversion operation prior to the verification operation. It may be noted that in the masked ECDSA, no matter how we choose t, we always have t=ck⁻¹. Since c is made public, t is not an independent variable.

While the invention has been described in connection with specific embodiments thereof and in specific uses, various modifications thereof will occur to those skilled in the art without departing from the spirit of the invention as set forth in the appended claims. For example in the above description of preferred embodiments, use is made of multiplicative notation, however, the method of the subject invention may be equally well described utilizing additive notation. It is well known for example that elliptic curve algorithm embodied in the ECDSA is equivalent of the DSA and that the elliptic curve analog of a discrete log algorithm that is usually described in a setting of, F*_(p) the multiplicative group of the integers modulo a prime. There is a correspondence between the elements and operations of the group F*_(p) and the elliptic curve group E(Fq). Furthermore, this signature technique is equally well applicable to functions performed in a field defined over F_(p) and F₂ _(n) . It is also to be noted that the DSA signature scheme described above is a specific instance of the ElGamal generalized signature scheme which is known in the art and thus the present techniques are applicable thereto.

The present invention is thus generally concerned with an encryption method and system and particularly an elliptic curve encryption method and system in which finite field elements are multiplied in a processor efficient manner. The encryption system can comprise any suitable processor unit such as a suitably programmed general-purpose computer. 

1. A method for verifying a signature for a message m in a data communication system established between a sender and a verifier, said sender having generated in a secure computer system a masked signature having a first signature component r computed using a first short term public key derived from a first short term private key; a second signature component s computed using a second short term private key on said message m, a long term private key, and said first signature component r; and a third signature component c computed using said first and second short term private keys, said method for verifying comprising said verifier: a) obtaining a regular signature derived from said masked signature (r, s, c), said regular signature having said first signature component r, and another signature component s computed using said second signature component s and said third signature component c; b) recovering a point on an elliptic curve defined over a finite field using said message m and said another signature component s; c) converting an element of said point to an integer; d) calculating a value r′ using said integer; and e) verifying said regular signature (r, s) if said value r′ is equal to said first signature component r.
 2. A method according to claim 1 further comprising the step of said verifier receiving said masked signature (r, s, c) from said sender and converting (r, s, c) to obtain said regular signature (r, s).
 3. A method according to claim 1 further comprising the step of said sender converting said masked signature (r, s, c) to said regular signature (r, s) and said sender sending said regular signature (r, s) to said verifier.
 4. A method according to claim 1 wherein said point is calculated using a pair of values u and v, said values u and v derived from said regular signature (r, s) and said message m.
 5. A method according to claim 4 wherein said point is calculated as (x₁, y₁)=uP+vQ, wherein P is a point on an elliptic curve E and Q is a public verification key of said sender derived from P as Q=dP.
 6. A method according to claim 4 wherein said value u is computed as u= s ⁻¹e mod n and said value v is computed as v= s ⁻¹r mod n, e being a representation of said message m.
 7. A method according to claim 6 wherein e is calculated as e=H(m), H( ) being a hash function of said sender and being known to said verifier.
 8. A method according to claim 1 wherein a coordinate x₁ of said point is first converted to an integer x ₁ prior to calculating said component r′.
 9. A method according to claim 8 wherein said component r′ is calculated as r′= x ₁mod n.
 10. A method according to claim 1 wherein prior to calculating said component r′, a coordinate pair (x₁,y₁) of said point is first verified, whereby if said coordinate pair (x₁,y₁) is a point at infinity, then said regular signature (r, s) is rejected. 